In recent years, hybrid vehicles are receiving widespread attention as environmentally friendly vehicles. As its motive power source, a hybrid vehicles further employs, in addition to a conventional engine, a power storage portion such as a rechargeable battery and an electric motor that operates by electric power from the power storage portion.
In a hybrid vehicle, the combustion efficiency is improved by optimally controlling torque and rotation speed of the electric motor. In order to realize such control of the electric motor, the hybrid vehicle includes a power converting portion such as an inverter. The power converting portion receives direct current (DC) power from the power storage portion, and converts it into alternating current (AC) power having prescribed voltage, frequency, phase and the like and supplies the result to the electric motor.
Heat is generated in the electric motor by copper loss attributed to resistance of the coil, or iron loss attributed to AC magnetic flux passing the core. Also, heat is generated in the power converting portion by switching loss in the switching elements such as IGBTs (Insulated Gate Bipolar Transistors). Here, the heat generation in the electric motor is chiefly attributed to copper loss, which increases along with the output of the electric motor. On the other hand, switching loss in the power converting portion is substantially constant irrespective of the electric power, provided that the switching frequency is the same. That is, while the heat generation amount in the electric motor changes in accordance with the generated vehicle drive force, the heat generation amount in the power converting portion hardly changes.
Thus, in the hybrid vehicle, it is necessary to cool the electric motor and the power converting portion, in addition to the engine. Then, for example, Japanese Patent Laying-Open No. 2004-112855 discloses a series-type hybrid vehicle that dispenses with an electric motor for driving a refrigerant press-feeding pump, so that the costs can be reduced and the weight of the vehicle is suppressed. The hybrid vehicle includes: a power generator coupled to the output shaft of an engine; a first refrigerant press-feeding pump coupled to the shaft of the power generator to pressurize and feed a refrigerant; and a second refrigerant press-feeding pump coupled to the shaft of the power generator to pressurize and feed a refrigerant. The refrigerant having been pressurized and fed by the first refrigerant press-feeding pump cools the electric motor, and the refrigerant having been pressurized and fed by the second refrigerant press-feeding pump cools an inverter.
It is to be noted that the series-type hybrid vehicle is a vehicle in which the electric motor powered by the power generator coupled to the output shaft of the engine solely generates the vehicle drive force, while the engine does not directly generate the vehicle drive force.
The configuration disclosed in Japanese Patent Laying-Open No. 2004-112855 is predicated on a series-type hybrid vehicle, i.e., predicated on the condition that the engine and the power generator always operate while the vehicle is traveling. However, many hybrid vehicles in practical use employ the parallel type configured to be capable of generating the vehicle drive force from any of the engine and the electric motor, or the parallel/series type in which elements of the series type is added to the configuration of the parallel type, in order to realize higher combustion efficiency (mileage). In such a parallel-type or parallel/series-type hybrid vehicle, the engine intermittently stops in accordance with a traveling situation. Accordingly, it has not been possible to apply the configuration disclosed in Japanese Patent Laying-Open No. 2004-112855 to parallel-type and parallel/series-type hybrid vehicles.